Moderate

Crossing over takes place during _ of meiosis.

Correct answer: A. Prophase I

  • A. Prophase I
  • B. Telophase I
  • C. Metaphase I
  • D. Anaphase I

Explanation

Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase). Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids. A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage:Prophase I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs.Crossing over occurs in Prophase I when homologous chromosomes pair up, forming a tetrad, and then non-sister chromatids exchange genetic material at points called chiasmata. This genetic exchange, known as recombination, shuffles alleles and creates new combinations of genes on the chromosomes. The process involves enzymes breaking DNA at similar locations on each chromosome and then reattaching the broken ends in a criss-cross pattern, resulting in genetically unique gametes. Metaphase I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell.Anaphase I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell.Telophase I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells.

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